Photosynthesis by plants is an important scientific concept that may not be taught under the new curriculum, its critics say Photograph: Murdo Macleod

The secondary school curriculum is "too vague" and no longer ensures pupils learn about important scientific concepts such as photosynthesis, leading exam assessors have warned.

Cambridge Assessment – part of Cambridge University and the parent organisation of OCR, one of England's main exam boards – has attacked the government for what it sees as a clouding of the curriculum.

In a two-page document, seen exclusively by the Guardian, the assessors criticise the removal of specific references to topics such as photosynthesis in the curriculum.

The references have been replaced by generalised statements about the scientific knowledge pupils must acquire.

Cambridge Assessment argues that it is no longer clear to teachers, pupils or parents what children need to be taught. The curriculum risks being "open to considerable variation and interpretation".

Changes implemented last year saw the number of bulletpoints detailing what scientific knowledge pupils aged between 11 and 14 should acquire drop from 94 to 14.

The 1999 curriculum states that pupils must be taught "the reactants in, and products of, photosynthesis", be able to summarise photosynthesis in a word equation and learn "that the rate of photosynthesis may be limited by light intensity, carbon dioxide concentration or temperature".

The new curriculum states only that pupils must understand that "life processes are supported by the organisation of cells into tissues, organs and body systems". It does not mention photosynthesis.

The number of bulletpoints detailing what mathematical knowledge pupils aged between 11 and 14 should acquire has fallen from 134 to 58 in the new curriculum, and does not include any statements about fractions or gravity.

Cambridge Assessment says: "There have been successive revisions to ... the national curriculum since its introduction two decades ago, and we are very concerned about the current direction of revision policy."

Rather than listing key concepts and processes, it says, the new curriculum "tends to focus on generalised statements of scientific activity and application. This ... threatens the notion of a clear, but succinct, statement of a common core of learning."

The change in content is partly a general move towards less government prescription over what pupils should be taught and is in response to teachers' complaints that the curriculum is overcrowded and lacks flexibility.

Tim Oates, Cambridge Assessment's group director of assessment, research and development, said: "The curriculum needed to be slimmed down. But it has been done in the wrong way: research shows that the most effective curricula are sparsely stated, focused on key concepts and then giving freedom to schools to design teaching around these concepts. We have not got that. The new curriculum is too vague."

But some teachers said they were happy with the changes. Sue Kirkham, education policy specialist at the Association for School and College Leaders, said: "People might assume that because every word of the old curriculum is no longer spelled out, that teachers will not be teaching it. But this is far from the case. The general reaction to the new curriculum has been very positive, and teachers are using the flexibilities in very creative ways."

In other subjects, such as English, the government has remained prescriptive and insisted that lists of classic authors are kept in the curriculum.

A DCSF spokesperson said: "This was a wide consultation that involved many stakeholders, and teachers were clear that they wanted us to free up the curriculum. That's what we have done with the updated curriculum. It's being phased in at the moment and it's been extremely well-received by teachers and schools."